1. Technical Field
The present invention relates to methods and apparatus for measuring and testing, and more particularly to measuring volume and mass rate of fluid flow. Still more particularly, the present invention relates to such method and apparatus which measure fluid flow by means of thermal sensing and resistive elements in a bridge circuit.
2. Background Information
To maximize efficiency in many internal combustion engines and other types of equipment, it is desirable that air inlet manifolds, and other fluid flow ducts, be equipped with sensors to measure fluid flow into the manifold. The prior art teaches numerous ways to measure fluid flow, which include vane anemometers, thermal hot wire anemometers and total pressure tubes. Thin film thermocouple rakes allow the sensor to be attached to a thin airfoil. While the prior art devices allow measurement of air intake flow rates, they do have certain limitations.
Hot wire anemometers are fragile and difficult to repair. Pitot tubes require intrusive piping into the manifold. Neither are able to determine the direction of air flow. Vane anemometers are subject to errors caused by the motion of the vehicle. Thin film thermocouple rakes on airfoils eliminate many of these problems, but require integration of signal to determine total flow. Fabrication of the two metal thermocouple rakes also requires two steps for fabrication.
Many of these disadvantages are overcome by an air flow sensor of the type shown in U.S. Pat. Nos. 5,629,481 and 6,131,453. These two patents use heated resistors and temperature sensors which connect to a Wheatstone resistance bridge to provide a bidirectional air flow sensing device which is based upon the sensed temperature differential as the air flow moves over heating elements. The various elements form legs of a Wheatstone bridge, the output of which provides the desired measurements. These prior art flow devices use substrates having the various sensing legs and components applied thereto by various techniques to provide for a compact, low cost and efficient device. Although these prior art devices may provide satisfactory results, the signal is reduced by the considerable parasitic resistance inherent in the design. More importantly, they measure only a relatively small portion of the flow stream, this may not provide the same accuracy as a sensor whose measurements are taken across a larger part of the flow stream.
Thus a need exists for an improved sensor for measuring manifold input flow rates.
It is an object of the present invention to provide a fluid flow sensor which reliably and cost effectively measures the mass flow of a fluid moving through a manifold.
It is a further object of the present invention to provide a fluid flow sensor which minimizes flow disturbance by locating the sensing elements on an airfoil substrate sufficiently downstream from the leading edge of the substrate containing the sensor elements so that the leading edge disturbance has dissipated.
It is a further object of the present invention to provide a fluid flow sensor which is simple to fabricate in one step requiring no etching or multiplicity of layers, thereby reducing the cost thereof, and which can be easily scaled to almost any desired size for a particular application.
It is a still a further object of the present invention to provide a fluid flow sensor which simplifies signal conditioning, and in which the parasitic resistance is very low since almost all of the resistance of the sensors participate in the measurement, thereby increasing signal to noise ratio.
Another feature of the present invention is that the airfoil design is such that the measured flow is the same as the actual upstream flow.
A further feature of the design is the ability to fabricate the sensor with half of the bridge circuit on each side of the airfoil or substrate, making alignment with the airflow less critical. In addition, temperature sensors may be mounted on the substrate, allowing the determination of the temperature of the gas, necessary to accurately measure mass flow.
These and other objects are met by the present invention which is a sensor for measuring mass flow in a fluid stream having an upstream direction and an opposed downstream direction. This sensor includes a first resistor whose function is to measure the temperature of the gas is positioned in the fluid stream.
A fluid heater is positioned in the fluid stream in downstream relation to the first resistor.
A second resistor is positioned in the fluid stream downstream in relation to the fluid heater. Its function is to measure the gas temperature if the flow has reversed.
Also encompassed by the present invention is a sensor array surrounding the heater for measuring mass flow in a fluid stream having a downstream direction in the direction of mass flow and an upstream direction opposed to the direction of mass flow. It also assumes an excitation voltage source having positive and negative poles. The sensor is arranged as a Wheatstone bridge comprising positive and negative excitation ports and a first and second signal port and first and second parallel pair of resistors. One of the first parallel pair of resistors is interposed between the first positive excitation port and the second signal port, and the other of said first parallel pair of resistors is interposed between the first negative excitation port and said first signal port. The first pair of resistors is positioned in the fluid stream in upstream relation to the fluid heater, and one of said second parallel pair of resistors is interposed between the second positive port and first signal port. The other of said parallel resistors is interposed between said second negative excitation port and second signal port and said second pair of resistors are positioned in the fluid stream in downstream relation to the fluid heater.
It is understood that there is an external voltage difference circuit (differential amplifier) having two input terminals and an output terminal. The two input terminals of the circuit are connected to the signal ports of the sensor. The output of voltage difference circuit is proportioned to the difference in voltage of the two signal ports of the sensor, which, in turn, is proportioned to the mass flow of the fluid in the ducts.